electrical question about charging..resistor values..limiting current.etc

Clint · #1 03-06-2003, 01:47 PM

each little nimh 1.2v 150mA batteries say they want to be charged at1.5v, 15mA for 16hours(online pdf specs)..instead of buying a peak charger...which i might do tomorrow anyway..ive been thinking about just slow charging the batteries manually.
will slow charging a fully dishcharged battery be similar to charging them in an r/c peak charger? ie ..will i get more runtime. i'm doing a balsa rc plane and i need all the runtime i can get from each batt.

i have a small 100mA power supply that does 3v and 4.5v, 6v, 7.5v, 9v @ 100mA

i want to slow charge a dual cell which is 2.4v and a 3 cell which is 3.6v
can i use the LED resistor principle to limit the current to 15mA..ie

can i use?
limiting resistor=(voltage of power source - voltage of battery) / desired mA)*1000

so for a 16 hour charge of a dual cell
((3v-2.4v)/15mA)*1000=40Ohm limiting resistor

for a 16 hour charge of a triple cell
((4.5v-3.6v)/15mA)*1000=60Ohm limiting resistor

for a 16 hour charge of a quad cell
((6v-4.8v)/15mA)*1000=80Ohm limiting resistor

discharging is done automatically by the bits, since u cant turn the cars off..they just drain off in an hour or two naturally. i know i'll need a switch so the charger doesnt power the board while slow charging..that would just be wasted effort.

Namuna · #2 03-06-2003, 02:52 PM

woohoo! Electronics talk!

First off, that 16 hours charging time seems just a bit...DRASTIC. I'm not going to discourage you from it, since I've never actually left something for such a long period of time to charge, so maybe it WILL make a big difference.

Ohm's Law - V = I * R

So, if you want 3v @ 15mA, then:

3 = .015 * R
R = 3 / .015
R = 200 ohms

4.5v @ 15mA = 300 ohms
6v @ 15mA = 400 ohms

As I understand it, a resistor ONLY effects the current, so you can't subtract the voltage differences...Besides, you NEED more than the rated voltage to charge anyway (like a 2.4v pack might need 2.8 or 2.9v to actually start charging)

hogjowlz · #3 03-06-2003, 02:57 PM

you are putting too much thought into this. ive been running the same batteries for months and have not noticed any decrease in capacity. when i do ill sport 6bucks and buy two more.

the specs you have read are for expensive batteries that are impossible to replace.

#4 03-06-2003, 03:49 PM

I got an AC battery charger from All Electronics. The output is 4.8v @ 350mA. They say it's great for charging 4 NiMH cells. Would this be good (or maybe better) for charging a tri-cell?

hogjowlz · #5 03-06-2003, 03:50 PM

ya. it would work

#6 03-06-2003, 03:59 PM

Thanks. Maybe I should fit a fourth cell somewhere into the chasis just because I can!

hogjowlz · #7 03-06-2003, 04:02 PM

i have enough trouble controlling 2 batteries.

dougal · #8 03-07-2003, 12:49 PM

Okay, here's a quick battery question:

I know that you can put batteries in series for more voltage, and in parallel for more current. If I put two batteries in series, then wire that in parallel with a third battery, I should double both voltage and current, right? (well, assuming tha they're all the same specs).

I was thinking that this would be a quick and easy way to multi-cell a car. Just make an external dual-cell pack, tape it to the roof of a car, then run leads into the chassis to put that pack in parallel with the existing battery.

Any major flaws in that idea?

hogjowlz · #9 03-07-2003, 12:54 PM

it wouldnt double the amps. i have a car wired up so it can be either series or parallel. you can charge a parallel circuit with a normal controller and switch it back to series for the power. also you can just wire in series and ad a 3rd battery to the controller.

parallel circuits give no performance boosts actually they decrease because of the weight.

dougal · #10 03-07-2003, 01:16 PM

Sorry, I mis-spoke. It wouldn't double the amps, but it would double the current capacity, right? So you'd get longer run times.

Namuna · #11 03-07-2003, 01:20 PM

dougal,

1- You are correct, parallel would increase runtime
2- You SHOULDN'T try to connect the 3rd battery in parallel, that's a mismatch. Once 2 batteries are connected, then it's at 2.4v. You should ONLY match like batteries...So putting a 1.2v in parallel would be a BAD IDEA.

If you were to take 4 batteries, connect each pair in series for 2 sets of 2.4v and THEN connect in parallel...that's okay.

hogjowlz · #12 03-07-2003, 01:21 PM

a stock bit is about 20grams. a bit with 4 batteries is about 30grams. thats a lot more weight.

Namuna · #13 03-07-2003, 01:33 PM

Yeah, it sure as heck would weigh down a Bit.

Anyway, here's a diagram, to illustrate what I was saying about connecting in Parallel

hogjowlz · #14 03-07-2003, 01:44 PM

another way to increase battery life is to decrease drive train friction. look at the "testbench" thread and notice the car with the epoxy in the rear. that has totally decreased any drive friction that i had. it runs super smooth and quiet. i bet i have at least a 30% increase in runtime since doing that. and a lot more acceleration and torque. it also coasts a lot further.

this was originally to fix my axle from falling out of the holder. then i realized it makes it way better than stock.

dougal · #15 03-07-2003, 02:08 PM

Quote:

Originally posted by Namuna

2- You SHOULDN'T try to connect the 3rd battery in parallel, that's a mismatch. Once 2 batteries are connected, then it's at 2.4v. You should ONLY match like batteries...So putting a 1.2v in parallel would be a BAD IDEA.

Define "bad".

Bad as in "permanently ruin your batteries" bad?